FLEXURAL STRENGTH OF DENTURE BASE ACRYLIC RESINS PROCESSED

BY CONVENTIONAL AND CAD/CAM METHODS

A Thesis

by

BRIAN C. AGUIRRE

Submitted to the Office of Graduate and Professional Studies of

Texas A&M University

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Chair of Committee, William W. Nagy

Committee Members, Elias Kontogiorgos

David Murchison

Jenn-Hwan Chen

Head of Department, Larry Bellinger

May 2017

Major Subject: Oral Biology

Copyright 2017 Brian Aguirre

ii

ABSTRACT

High flexural strength is a desirable property for denture base resins. Currently

there is a lack of published studies evaluating the physical properties of newer denture

bases such as the CAD/CAM milled bases. The purpose of this study was to compare the

flexural strength of three different denture base resins fabricated by compression

molding, injection molding, and pre-polymerized CAD/CAM milling. Three groups of

ten PMMA acrylic denture base resins were processed into rectangular plates for total

sample size of thirty (n=30). The groups differed in their method of processing and resin

composition. The three groups were injection molded (SR-Ivocap High Impact, Ivoclar

Vivadent), compression molded (Lucitone 199, Dentsply), and pre-polymerized

CAD/CAM milled resins (Avadent Digital Dentures). Following storage in water for one

week, flexural strength was measured using a 3-point bend test until failure. Kruskal-

Wallis and Mann-Whitney tests were used for statistical comparison between groups.

Significant differences in flexural strength was observed among the groups

tested. The flexural strength of the pre-polymerized CAD/CAM milled acrylic resin

group was higher than that of conventional method groups of compression molded and

injection molded. The compression molded group exhibited higher flexural strength than

the injection molded group. The results suggest that pre-polymerized milled denture

bases may be a useful alternative to conventionally processed denture bases in situations

where increased bending forces are anticipated.

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ACKNOWLEDGEMENTS

I would like to thank my thesis committee members; Dr. William Nagy, Dr. Elias

Kontogiorgos, Dr. David Murchison, and Dr. Ken Chen for their guidance and support.

I’d like to especially thank my director, Dr. Nagy, for his guidance, patience, and

support throughout residency and in completing this thesis.

Thanks to me loving family, Amber and Jackson, for being patient with me and

allowing me to pursue my dreams. To my parents, Velma and Ruben, I wouldn’t be

where I am today without your support throughout my life. Lastly, to my co-residents,

thank you for all the good times and always pushing me to take it the next level.

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CONTRIBUTORS AND FUNDING SOURCES

This work was supervised by a thesis committee consisting of Dr. William Nagy,

Dr. Elias Kontogiorgos, and Dr. Ken Chen of the Department of Restorative Sciences

and Dr. David Murchison of the Department of Diagnostic Sciences.

Statistical analysis of data was conducted in part by Dr. Elias Kontogiorgos. All

other work conducted for the thesis was completed by the student independently.

Funds were provided by the Office of Research and Graduate Studies at Texas

A&M University College of Dentistry.

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TABLE OF CONTENTS

Page

ABSTRACT .......................................................................................................................ii

ACKNOWLEDGEMENTS ............................................................................................. iii

CONTRIBUTORS AND FUNDING SOURCES ............................................................. iv

TABLE OF CONTENTS ................................................................................................... v

LIST OF FIGURES ........................................................................................................... vi

LIST OF TABLES ...........................................................................................................vii

1. INTRODUCTION .......................................................................................................... 1

2. MATERIALS AND METHODS ................................................................................... 8

2.1 Compression molded fabrication ............................................................................. 8

2.2 Injection molded fabrication .................................................................................... 9 2.3 Pre-polymerized CAD/CAM milled ...................................................................... 10 2.4 Preparation of samples for flexural strength test .................................................... 10

2.5 Three-point bend test and measurement of flexural strength ................................. 10

2.6 Calculations and statistical analysis ....................................................................... 11

3. RESULTS ..................................................................................................................... 13

4. DISCUSSION .............................................................................................................. 16

5. CONCLUSIONS .......................................................................................................... 20

REFERENCES ................................................................................................................. 21

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LIST OF FIGURES

Page

Figure 1. Three-Point Flexural Test Setup ....................................................................... 11

Figure 2. Deformation of Injection Specimen (Right) Compared to Milled Specimen

(Left) ................................................................................................................. 14

Figure 3. Mean Flexural Strength Bar Graph ................................................................... 15

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LIST OF TABLES

Page

Table 1. Descriptive statistics examining flexural strength (MPa) for the 3 groups

(Injection, compression, milled). ...................................................................... 14

Table 2. Mann-Whitney Test for differences between 2 groups according to flexural

strength. ............................................................................................................ 14

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1. INTRODUCTION

In order for a denture base to be successful, it should exhibit several important

qualities. According to Zarb, dental resins should be biocompatible, esthetic, cleansable,

easily repairable, adhere to denture teeth, and have adequate physical and mechanical

properties. (1) It also needs need to be fabricated in an efficient, yet economical manner.

Denture base resins should have enough strength and toughness to stand up to forces

generated during function while also being dimensionally stable for many years under

varying thermal conditions. (2) High flexural strength is critical due to the uneven force

distribution the base will endure as the alveolar ridge irregularly resorbs. Therefore, it

should be able to resist plastic deformation and fatigue resistance under repeated loads.

Historically denture bases were fabricated from materials such as wood, ivory,

and bone. Dental polymers were introduced in 1853 in the form of vulcanized rubber, a

plant-derived latex. (2) This material was widely used until the introduction of

polymethyl-methacrylate (PMMA) in 1936 which remains the denture base material of

choice today. PMMA is the polymer of methyl methacrylate with a chemical formula

C5H8O2. It is a colorless polymer that goes by the common name of acrylic. PMMA has

many physical and mechanical properties which make it a suitable denture base material.

It’s one of the hardest thermoplastics with a high mechanical strength and modulus of

elasticity. Additionally, the low water absorption and dimensional stability over time

makes PMMA a capable material for the dynamic oral environment.

Numerous acrylic resins have outperformed PMMA, but most have not made it

to clinical dentistry due to difficulties in processing and higher costs. (3) The chemistry

2

of PMMA has evolved over the years. In addition to alteration of polymer chain

configurations, multifunctional monomers have been utilized to enhance crosslinking.

(2) Although these enhancements have greatly improved several properties of PMMA, it

still has the inherent disadvantage of having a relatively low strength. Denture base resin

strength is dependent on a number of factors and is non-linear due to plastic deformation

that occurs. (2) Clinically, this translates to deformation of the base under heavy loads.

While some of the deformation is recovered, some permanent deformation remains.

Several PMMA denture base resin types are available today. Their compositions

are similar but small variations lead to different physical properties and processing

methods. (2) They’re provided in a powder and liquid and used in a ratio of 3:1. The

powder typically consists of pre-polymerized PMMA and an initiator, benzoyl peroxide.

Additionally, it includes opacifiers, pigments, and glass or nylon fibers to give the

PMMA the desired esthetic. The basic liquid consists of methyl methacrylate monomer

with an inhibitor, hydroquinone, and a plasticizer, Dibutyl phthalate. Manufactures will

combine different monomers and polymers to produce a product with their desirable

properties using copolymerization. High impact resistant resins incorporate rubbery

comonomers such as butyl acrylate that results in dispersion of rubber inclusions.

Therefore, decreasing likelihood of fracture if dropped.

In heat activated resins, thermal energy activates benzoyl peroxide which

initiates the polymerization process. The heat may be provided via water bath or

microwave energy. Alternatively, the activator can also be a chemical rather than heat.

Chemically activated denture base resins, or cold-cure resins, do not require thermal

3

energy but use tertiary amines to activate benzoyl peroxide. Generally, the degree of

polymerization reached using chemically activated resins is lower than heat activated

resins. This yields excess residual monomer which acts as a plasticizer negatively

affecting the physical properties. It’s been demonstrated that dentures made in this

manner have lower mechanical properties than those made with heat activated resins. (4)

Another type of denture base resin used is light activated denture base resins. These have

a matrix of urethane dimethacrylate, microfine silica, and acrylic resin monomers.

Visible light activates camphorquinone which serves as the initiator for polymerization.

Diaz-Arnold et al. demonstrated these resins have superior flexure strength than all the

heat polymerized resins tested but demonstrated greater standard deviations and

brittleness. (5)

CAD/CAM (Computer-Aided Design/Computer Aided Manufacturing) was

introduced in the late 1950’s with the introduction of PRONTO, a numerical control

programming tool. This was the first CAM software system developed by Dr. Patrick J.

Hanratty, considered the father of CAD/CAM. (6) Several developments in CAD/CAM

technology for dentistry occured in the 1980’s with the help of three pioneers who

shaped the current methods used today. Two of them, Dr. Moermann and Dr. Andersson,

went on to develop two modern CAD/CAM systems CEREC® and Procera® for clinical

dental restorations. (7) Since the turn of the century, this same technology has evolved to

include fabrication of complete dentures. The bases are milled from pre-polymerized

resin pucks which provide superior strength and fit, reduced bacterial adhesion, and

reduced cost to patient and clinician. (8)

4

Several studies have examined the incidence and causes of denture fractures with

broken dentures reported as high as 63% in the first 3 years of service. (9) There is a

general trend towards more fractures occurring in the maxilla with an approximate ratio

of 2:1 compared to the mandible. (10, 11) Generally, fractures result from either flexural

fatigue or impact forces. (3) Fracture due to flexural fatigue is usually explained by the

development of microscopic cracks in a stress concentration area. This type of fracture

occurs over time and is not due to a single application of force like a fracture due to

impact. Stress distribution within dentures has been comprehensively investigated via

many methods including finite element modeling, photo-elastic analysis, strain gauges,

and holography.(12-15) Maxillary dentures are prone to tensile stresses in the incisor

region, both labial and lingual, due to bending deformation. (16) The incisal notch and

hard palatal midline are at the center of these concentration points as ridge resorption

occurs. Midline fractures have been reported as the second most common fracture site

behind denture teeth. (17) There are other contributing factors which may leave a

denture more susceptible to fracture. Finite element studies have shown occlusal scheme

and location of contacts play a role in maxillary denture fractures. (18) Additionally;

diastemas, palatal tori, and sharp frenal notches create localized stress areas causing

midline fractures. (9, 15, 19)

To resist these aforementioned complications, there are a number of mechanical

properties of interest due to the various stresses these resins may undergo. Denture bases

undergo repeated flexing during mastication for several years. Often times this may lead

to fatigue failure of the prosthesis. (20) Therefore, a high fatigue strength is a desirable

5

property of denture base resins. Testing fatigue strength is a laborious and very time

consuming process compared to measuring other properties. In the event of dropping the

prosthesis extra-orally, impact strength becomes critical. This property is the capability

of the material to withstand a suddenly applied load. A popular method to measure

impact strength is an Izod impact test which uses a single high speed of impact to

fracture the sample. Disadvantages of this method are the extrinsic variables inherent in

the testing, such as, specimen dimensions, notch depth and radius, impact velocity, and

other factors.(21) This suggests that the impact test is better suited for prosthesis design

rather than comparing intrinsic properties of materials. Since fracture is frequently

preceded by crack formation, a higher fracture toughness is also desirable. (22) This is

the ability of the material to resist crack propagation, a critical property for a denture

base to possess. Some denture base resins fracture in a smooth, rapid, fashion while

others may fracture slowly with a permanent deformation and irregular fracture surface.

(23) Flexural strength represents the highest stress experienced within the material at its

moment of rupture. Transverse strength is another term used to describe flexural

strength, though they are measured the same way. Given the intraoral function of the

denture base, a high flexural strength is required to prevent catastrophic failure under

load. The ability to withstand high flexural loads is paramount for the success of a

denture. One example is in the implant fixed complete denture (IFCD) provisional where

the denture is subjected to high flexural loads on either side of each implant. Similarly,

in the implant retained overdenture, the remodeling of the alveolar ridge may lead to

6

increased flexural loads as the denture fulcrums over the abutments. Frequently, flexural

strength is measured utilizing a three-point flexural test.

The measurement of the physical and mechanical properties of denture base

resins is dependent on several extrinsic factors as well. American Dental Association

Standard No. 139 in accordance with ISO 20795-1 for denture base polymers describes

the effect of time and temperature during polymerization and during testing. (24) Due to

variations in the formulation of different resins, manufacturer’s instructions for mixing

time and temperature must be followed carefully. Also, when running tests, such as the

three-point bend, the medium in which it’s bent should reproduce the oral environment.

Specimens flexed in an aqueous environment exhibit lower strength compared to

performing test in air. Additionally, to simulate the oral cavity, conditioning of the resin

prior to testing is necessary. It’s been demonstrated thermocyling negatively effects

flexural strength of denture base resins. (25)

Several studies have compared the flexural or transverse strength of different

types of denture base resins and their various processing methods. (5, 23, 26-31) The

compression and injection molded are the most popular studied and employed methods

used. Lucitone 199 is one of the most studied resins. It has been shown to have superior

flexural strength in some studies and is highly regarded in the dental community. Ivocap

High impact (Ivoclar Vivadent) is another frequently used resin in combination with

injection molding due to its improved accuracy of fit. (2)

The purpose of this study was to compare the flexural strength of three different

denture base resins fabricated by compression molding and injection molding, and pre-

7

processed CAD/CAM milling. The null hypothesis was that there is no significant

difference in the flexural strength among the three groups.

8

2. MATERIALS AND METHODS

Pre-polymerized milled rectangular plates (64mm x 10mm x 3.3mm) were ordered

from Avadent Digital Dental Solutions. A vinyl polysiloxane putty (3M/ESPE) matrix

was created from these specimens. Pink base plate wax was then dripped into the matrix

to form wax duplicates. Three groups of ten PMMA acrylic denture base resins were

processed into rectangular plates in accordance to ISO 20795-1 (64mm x 10mm x

3.3mm) for total sample size of thirty. The groups differed in their method of processing

and resin composition. The three groups were injection molded (SR-Ivocap High Impact,

Ivoclar Vivadent), compression molded (Lucitone 199, Dentsply), and pre-polymerized

CAD/CAM milled (Avadent Digital Dentures) resins. Plates were finished, polished and

stored in distilled water for one week. Flexural strength was measured in a Universal

Testing Machine (Instron Ltd.) using a three-point bend test.

2.1 Compression molded fabrication

Wax plates (64mm x 10mm x 3.3mm) were flasked and invested according to

manufacturer’s instructions in ISO type 3 dental stone (Whip Mix Corp.). The flask was

heated in boil out solution for 8 minutes, separated, and the wax flushed with boiling

solution (Patterson Dental). The final flush was done with clean water and the halves

were allowed to cool to room temperature. ALCOTE Separator (Dentsply) was applied

to the stone and allowed to dry. Lucitone 199 Resin (Denstply) was mixed with 21g

polymer to 10ml monomer to assure wetting of all polymer particles. The jar was

covered and material allowed to reach packing consistency, not sticky or rubbery (10

9

minutes). Resin was condensed into the mould with finger pressure and the flask was

closed in a Pneumatic Flaskpress (Coe-Bilt) under 6000 pounds of pressure. The flask

was then loaded in a spring clamp and placed in a Hanau Curing Unit (Whip Mix) for 9

hours at 163oF, followed by 30 minutes in boiling water (2120F) per manufacturer

instructions. Bench cooling was allowed for 30 minutes and then the flask was

immersed in 70oF water for 15 minutes prior to deflasking.

2.2 Injection molded fabrication

Wax plates (64mm x 10mm x 3.3mm) were flasked and invested according to

manufacturer’s instructions in ISO type 3 dental stone (Whip mix Corp.). The flask was

heated in boil out solution (Patterson Dental) for 8 minutes, separated, and the wax

flushed with boiling solution. The final flush was done with clean water and the halves

were allowed to cool to room temperature. Separating fluid (Ivoclar Vivadent) was then

applied to the stone and allowed to dry. Premeasured capsules of resin and monomer

(SR-Ivocap High Impact, Ivoclar Vivadent) were combined and mounted in the Cap

Vibrator (Ivoclar Vivadent) for 5 minutes. The flask halves were clamped in a clamping

frame under 6000lbs of pressure. The mixed capsule was inserted into the flask and the

SR Ivocap pressure apparatus was attached. The pressure apparatus was connected to a

compressed air supply (6 bar/ 85 psi) to allow the plunger to descend and press material

into the mould. Ten minutes of injection time was programed at room temperature. The

assembly was then placed in the polymerization bath at the appropriate water level.

Polymerization was allowed to occur for 35 minutes under boiling water. The assembly

10

was then removed and immediately placed in cold water, maintaining pressure, for 30

minutes. The specimens were then deflasked.

2.3 Pre-polymerized CAD/CAM milled

Pre-polymerized CAD/CAM milled rectangular plates were provided from Avadent

Digital Dentures and were in final dimensional specifications.

2.4 Preparation of samples for flexural strength test

After processing, samples were evaluated to ensure absence of voids or gross

irregularities with 3.5x magnification. The samples were then finished with wet 220 to

600 grit silicon carbide paper (3M ESPE) to a final dimension of 63 x 10 x 3.3mm as

measured with digital caliper (Neiko) at 5 points to +-.03mm. Prior to flexural strength

test, all samples were stored in distilled room temperature water one week for

conditioning.

2.5 Three-point bend test and measurement of flexural strength

Samples were tested using a three-point bend test per guidelines of ISO 20795-1 for

denture base polymers. Each sample was placed on circular support beams with a 50mm

span. (Figure 1) A load was applied with a Universal Testing Machine (Instron) to the

center of the samples at a crosshead speed of 5mm/min until fracture. The moment of

fracture was designated as the moment applied load dropped to zero. Data was recorded

on Bluehill Sofware (Ver 1.5).

11

Figure 1. Three-point Flexural Test Setup

2.6 Calculations and statistical analysis

Maximum load exerted was recorded in Newtons (N). Flexural strength (Fs) was then

calculated:

Flexural strength (Fs) = 3PL/2bd2

P = maximum load b = specimen width

L = span length d = specimen thickness

Data was analyzed using statistical software (SPSS 19.0, SPSS Inc). Kruskal-

Wallis test evaluated the differences between sample groups (Packed, Injected, Milled)

12

with a significance level of p≤.05. The Mann-Whitney test was performed when a

difference was detected between groups with an adjusted significance level set to

p≤0.017.

13

3. RESULTS

The flexural strength of the pre-polymerized CAD/CAM milled acrylic resin

group was higher than that of compression and injection molded groups. The

compression molded group exhibited higher flexural strength than the injection molded

group. The Mann-Whitney test showed that the difference was statistically significant

(P=0.001) between the three groups.

The milled and compression molded resin groups fractured with minimal to no

plastic deformation in contrast to the injection molded resin that displayed pronounced

deformation. This could be visualized by re-approximating the specimens following

fracture. (Figure 2)

The data for the means of flexural strength of the different groups are shown in

Table 1 and Figure 3. The mean value (SD) for the pre-polymerized milled samples was

the highest with 145.61 MPa (6.58), followed by 116.61 MPa (3.14) for the compression

molded group and 86.73 MPa (7.06) for the injection molded group.

Table 2 shows statistical differences between each specific group. When the

injection molded group was compared to the compression and pre-polymerized milled

groups, significant differences were observed (p=0.001). Likewise, when comparing the

compression to the pre-polymerized milled groups, a significant difference was noted

(p=0.001).

14

Figure 2. Deformation of Injection Specimen (Right) Compared to Milled Specimen

(Left)

Table 1. Descriptive statistics examining flexural strength (MPa) for the 3

groups (Injection, compression, milled).

Injection Compression Milled

N 10 10 10

Mean 86.7341 116.6105 145.6141

SDa 7.06461 3.14359 6.57883

Minimum 76.51 110.11 135.52

Maximum 97.31 120.57 153.41 a. Standard Deviation

Table 2. Mann-Whitney Test for differences between 2 groups according to

Flexural Strength. Injection vs

Compression Injection vs Milled

Compression vs

Milled

Mann-Whitney U 0.000 0.000 0.000

Wilcoxon W 55.000 55.000 55.000

Z -3.780 -3.780 -3.780

Significance*(p≤0.017) 0.001 0.001 0.001

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Figure 3. Mean Flexural Strength Bar Graph

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4. DISCUSSION

The flexural strength of denture base resins is an important property because when

the flexural, or transverse, strength of a material is exceeded, you are also analyzing the

compressive, tensile, and shear strengths. Flexural strength is a combination of these

three mechanical properties. Subjecting a denture base to a three-point bend test

simulates its ability to succeed intra-orally under high functional loads during

mastication and parafunction. Therefore, numerous prior studies have used this test to

evaluate the suitability of novel denture bases. According to the international standards

for polymer materials and ISO 20795-1 for denture base polymers, the three-point

flexural test is the most common method for measuring flexural properties. The standard

states that acrylic resins should measure no less than 65 MPa. Thus, all samples in the

current study are suitable for clinical use.

The present study compared the mean flexural strength recorded with a universal

testing machine for three different resins processed by compression molding, injection

molding, and milled pre-polymerized CAD/CAM acrylic resin samples. The results

demonstrated significant differences in flexural strength among the three groups. This

supports rejection of the null hypothesis that there is no significant difference in the

flexural strength among the groups tested. While there is no published literature looking

at the pre-polymerized milled resin flexural strength, this data is in disagreement with a

previous study comparing compression and injection- molded resin. Gharechahi

concluded that injection-molded resins had a higher flexural strength than pressure-

packed in his study (26). While his methods were similar to the present study, the

17

compression molded group was processed with an acrylic from a different manufacturer.

These differences occurring in most published studies should be considered, as

manufacturers do not disclose the composition of their products.

Limitations of the current study were the lack of cyclic loading and thermocycling

prior to the three-point flexural test. Also, the samples tested do not reflect the shape of

an actual denture. The effect of thermocycling on the flexural strength of denture base

resins was examined in a prior study. Thermocyled (5000 cycles) samples of Lucitone

199 displayed significantly lower flexural strength compared to samples that were not

thermocycled. (25) This is due to the effect water has on the physical properties of

processed polymers. Structural changes occur to the polymer chains as water molecules

interfere, acting as plasticizers. As a substitute, the samples in the present study were

submerged in de-ionized water for one week to simulate the aqueous intra-oral

environment. Additionally, cyclic loading would fatigue the samples to better simulate

intra-oral conditions. Although a previous study showed no significant difference in

flexural strength between heat polymerized specimens that underwent cyclic loading

(10,000 cycles) or not. (5)

The higher flexural strength values of the CAD/CAM milled samples may be

attributed to the higher degree of polymerization. One of the major determinants of resin

strength is the degree of polymerization achieved. As it increases, so does the ultimate

strength of the resin. Using propriety methods, the CAD/CAM milled dentures are

milled from a solid pre-polymerized puck. It is assumed these pucks are polymerized to

a very high degree using equipment more sophisticated than conventional methods. As a

18

result, it yields a highly condensed and less porous resin. (8) Conversely, this is why

chemically activated, or cold-cure, resins exhibit decreased strength and density.

Differences in flexural strength may be credited to the different composition of the

polymer and monomers. Resins that claim to be “high impact” may incorporate rubbery

comonomers such as butyl acrylate that result in dispersion of rubber inclusions.

Consequently, the impact strength in increased. The compression and injection molded

groups in the current study claim to be high impact. However, it is unknown whether the

pre-polymerized milled resin group were. These resins may negatively affect the flexural

strength at the expense of increased impact strength due to increased flexibility. (3) An

assessment of the samples after fracture revealed much more noticeable permanent

deformation of the injection molded resin. This may be related to the amount of rubbery

comonomers present in this resin, however this isn’t disclosed by the manufacturer. The

SR Ivocap High Impact resin underwent substantially more permanent deformation prior

to fracture. Clinically this may lead to subclinical deformation of the denture rather than

fracturing under load. The pre-polymerized and compression molded resins displayed

more similar fracture characteristics, displaying minimal to no deformation.

This study investigated a specific mechanical property of three commercially

available denture base resins. No prior studies have been published examining the

mechanical properties of milled denture base resins. Future studies should evaluate other

mechanical properties such as the impact strength and fracture toughness of these pre-

polymerized milled resins. Avadent Digital Dental Solutions also mills monolithic

dentures where the teeth and base are milled from a single piece. They claim these

19

dentures can be up to 8x stronger than ones processed by conventional methods, yet this

has not been verified. Clinically, a resin with a higher flexural strength may be less

inclined to fracture during function. Thus, using a milled denture base may be beneficial

in cases where you anticipate heavier functional loads or a patient suffering from

multiple denture fractures not due to accidental drop. Due to cost restraints and ease of

conventional denture processing methods, this is not always feasible. Most clinical

situations are unique and therefore material selection, depending on the situation, is

paramount for denture success. While the flexural strength of the injection molded

samples was the lowest, it is still a suitable material and its properties may be

advantageous in some clinical situations. For instance, in implant fixed complete

dentures (IFCD), occlusal forces are much greater than in a removable complete denture

and commonly suffer from fractures. This is usually an emergency and a difficult

situation for both the clinician and patient. One solution is to provide a material with the

highest flexural strength and prevent the fracture all together. Alternatively, using a

material with a low modulus and ability to undergo permanent deformation may prevent

the fracture from ever occurring. Future studies may provide guidelines for material

selection in dentures for specific clinical situations.

20

5. CONCLUSIONS

Within the limits of this in vitro study, the flexural strength of a denture base resin

may be influenced by the method in which it is processed. Pre-polymerized CAD/CAM

milled denture resin exhibited higher flexural strength compared to the two conventional

processing methods, compression and injection molded. The milled and compression

molded resins fractured with minimal to no plastic deformation in contrast to the

injection molded resin that displayed pronounced deformation. The results suggest that

pre-polymerized milled denture bases may be a useful alternative to conventionally

processed denture bases in situations where increased bending forces are anticipated.

21

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